A METHOD OF SCALE DISSOLUTION IN A FORMATION CONTAINING MULTIPLE PRODUCTIVE INTERVALS

Abstract: A method to improve the efficiency of scale dissolution in a formation containing multiple productive intervals (20, 22, 24, 26) where variable density liquids are utilized. Initially, a liquid composition sufficient to solubilize an alkaline-earth metal sulphate scale deposit is directed into a wellbore (10) thereby contacting scale in one productive interval of the formation. Thereafter, a second scale solubilizing liquid having a density less than the first liquid is directed into the wellbore (10) thereby contacting additional perforations (14) and a higher interval. These scale solubilizing liquids with varying densities are allowed to remain in contact with perforations (14) and productive intervals (20, 22, 24, 26) communicating therewith for a time sufficient to solubilize the scale deposit thereby efficiently removing said scale deposit from perforations (14) and productive intervals (20, 22, 24, 26) at two different levels.

Description: A Method of Scale Dissolution in a Formation Containing Multiple Productive Intervals This invention relates to a method of scale dissolution in a formation containing multiple productive intervals. Most water contains alkaline-earth metal cations, such as barium, strontium, calcium and magnesium, and anions, such as sulphate, bicarbonate, carbonate, oxalate, phosphate, silicate and fluoride. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, precipitates form until their product solubility concentrations are no longer exceeded. For example, when the barium ion and sulphate ion exceed the solubility of the barium sulphate reaction product, a solid phase of barium sulphate will form as a precipitate. Solubility product concentrations are exceeded for various reasons, such as evaporation of the water phase, change in pH, pressure or temperature and the introduction of additional ions which can form insoluble compounds with the ions already present in the solution. As these reaction products precipitate on the surfaces of the water-carrying or water-containing system they form adherent deposits or scale. The scale prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbours bacteria. Scale is an expensive problem in many industrial water systems, in production systems for oil and gas, in pulp and paper mill systems, and in other systems, causing delays and shutdowns for cleaning and removal. Once-through and recirculating cooling water systems are subject to the formation of scale deposits. Waterside problems encountered in boilers and steam systems include the formation of scale and other deposits, corrosion and foam. Scale and other deposits on heat-transfer surfaces can cause loss of the thermal efficiency of the boiler and can make the temperature of the boiler metal increase. Under scaling conditions, temperatures may go high enough to lead to failure of the metal due to overheating. Barium and strontium sulphate scale deposits present a unigue and sometimes "insoluble" problem. Under most conditions, these sulphates are considerably less soluble in all solvents than any of the other commonly encountered scale- forming compounds. It is generally acknowledged that barium sulphate scale is almost impossible to remove by chemical means. Conseguently, barium sulphate must be removed mechanically or the eguipment, pipes, etc., containing the deposit must be discarded. The incidence of barium sulphate scale is worldwide, and it occurs principally in systems handling subsurface waters. The barium sulphate scale problem is of particular concern to the petroleum industry since increasing volumes of water are produced with petroleum and more petroleum is produced by the water-flooding method of secondary recovery. The scale may occur in many different places, including production tubing, wellbore perforations, the area near the wellbore, gathering lines, meters, valves and in other production eguipment. Deposition of scale in production facilities and formation channels is a well-known source of problems in oil recovery. Barium sulphate scale is particularly troublesome when sulphate- rich seawater is used as an injection fluid in oil wells whose formation water is rich in barium ions. This scale causes severe problems in US oil fields and older North Sea oil fields. Scaling of this nature is expected to occur during advanced production stages in other North Sea fields particularly after seawater breakthrough has taken place. Barium sulphate scale may also form within subterranean formations such as in disposal wells. Scales and deposits can be formed to such an extent that the permeability of the formation is impaired resulting in lower flow rates, higher pump pressures, and ultimately abandonment of the well. US-A-5093020 discloses a method and composition for removing alkaline earth metal sulphate scale deposits from wellbores and equipment used in the production of hydrocarbonaceous fluids from a reservoir or formation. The composition comprises an aqueous solution having a pH of about 8 to about 14, an EDTA or DTPA chelant, and a catalyst or synergist. Preferred chelants comprise diethylenetriamine- pentaacetic acid (DTPA) or ethylenedia inetetraacetic acid (EDTA) or alkali salts thereof. Although this method is effective in removing alkaline earth metal sulphate scale deposits from wellbores, precise placement of the composition into a desired interval of the wellbore could be improved. This is particularly true when diversion of this high density composition over an extensive interval is required in a hydrocarbonaceous fluid producing formation. If several feet of wellbore have been perforated for production, and if the perforated interval contains sections of high permeability (greater than 10 to 20 d) , it may be difficult to treat the entire zone effectively with this high density composition at low surface injection rates, i.e. about 1 to about 2 BPM. What is needed is a method that would permit distribution of a high density alkaline earth metal sulphate scale solubilizing composition over an extended perforated productive interval of a formation or reservoir. The invention seeks to provide this. According to the present invention there is provided a method of scale dissolution in a formation containing multiple productive intervals comprising: (a) directing into a perforated wellbore a first liquid composition, of a known density, for dissolving an alkaline-earth metal sulphate scale deposit from perforations and productive intervals communicating with said wellbore; and (b) directing into said wellbore, on top of said first liquid composition, a second liquid composition for dissolving an alkaline-earth metal sulphate scale in a higher perforated interval of said wellbore which communicates fluidly with said productive intervals, wherein said second liquid composition has a density less than that of the first liquid composition. Preferably, in step (b) , the density of the second liquid composition is decreased by adding a lower density salt solution of an alkali or alkaline-earth metal. More preferably, in step (b) , the density of the second liquid is decreased by adding an aqueous or hydrocarbonaceous solution containing a salt of an alkali or alkaline-earth metal in an amount sufficient to obtain a density of at least 0.1 wt% less than the density of the first liquid composition. It is particularly preferred that said aqueous or hydrocarbonaceous solution contains bromides or chlorides. More specifically, the aqueous salt solution may be selected from a member of the group consisting of sodium chloride, potassium chloride, zinc chloride, sodium bromide, potassium bromide, or zinc bromide. Desirably, the density of the first liquid composition is from 11.5 to 12.0 pounds per gallon (0.955 to 1.00 kg/dm). Desirably also, the density of the second liquid composition is from 10.5 to 11.0 pounds per gallon (0.872 to 0.914 kg/dm). If necessary, successive liquids with decreasing density of at least 0.1 wt% less than the liquid last added to the wellbore are added above the first and second liquids until all of the desired productive intervals and perforations communicating therewith have been treated with the scale solubilizing solution. For example, after step (b) , a third liquid composition for dissolving scale is directed into the wellbore which has a density from 9.5 to 10.0 pounds per gallon (0.789 to 0.831 kg/dm); thereafter, a fourth liquid composition for dissolving scale is directed into the wellbore which has a density from 8.5 to 9.0 pounds per gallon (0.706 to 0.748 kg/dm) . It is preferred that the first and second liquid compositions, and any successive compositions comprise an aqueous solution having a pH from 8 to 14, each solution containing: (i) an a inocarboxylic acid or a polyamine chelant, salts and mixtures thereof in an amount from 0.1 to 1.0M; and (ii) a catalyst in an amount from 0.01 to 0.5M which catalyst is a member selected from the group consisting of fluoride, oxalate, persulfate, dithionate, hypochlorite, formate, thio, amino, and hydroxy acetate anions, thereby making a solution which can dissolve substantially more scale within a substantially reduced time than is possible with said chelant alone. Preferred chelants comprise diethylenetriaminepentaacetic acid (DTPA) and ethylenediaminetetraacetic acid (EDTA) or salts and mixtures thereof. The density of the liquid compositions used herein is decreased by utilizing a lower density liquid preferably having a pH of at least about 12 that is mixed to obtain a liquid composition of a desired density. By decreasing steadily the density of the liquid compositions to solubilize the sulphate scale deposits, various portions of the perforated intervals can be treated to an extent previously unattainable by other methods. Thus, in the preferred embodiment, each liquid composition comprises a chelant, a catalyst and a lower density liquid which preferably has a pH of greater than 12. The invention makes possible the treatment of an extensive perforated interval of a wellbore communicating with multiple productive intervals so as to remove alkaline-earth metal sulphate scale and particularly barium sulphate scale therefrom. Reference is now made to the accompanying drawing which is a schematic representation of a well-bore containing first and second scale solubilizing liquid compositions of varying densities. As shown in the drawing, a first alkaline-earth sulphate scale solubilizing liquid composition is directed into wellbore 10 which penetrates a formation or reservoir 12. The wellbore 10 has perforations 14 therein at different levels or intervals of the formation 12. The perforations 14 fluidly communicate with different productive levels or intervals of the formation 12. The first scale solubilizing liquid composition which enters the formation is of a density higher than any of the compositions which will be subsequently injected into the formation. This highest density liquid which is first injected into wellbore 10 contains a composition which is sufficient to solubilize an alkaline-earth metal sulphate scale deposit from perforations in the wellbore and productive intervals communicating therewith. Once the first highest density liquid has been placed in the wellbore, a second liquid containing a scale solubilizing composition, similar to the first liquid composition, is injected into the wellbore 10. The second liquid composition makes possible the dissolution of an alkaline-earth metal sulphate scale deposit from perforations in the wellbore and productive intervals communicating therewith. The second liquid composition has a density lower than the first liquid composition injected into the wellbore. The density of the second liquid should be at least 0.1 wt% less than the first liquid composition. Because the second liquid composition has a density less than that of the second liquid composition, it will remain above the first liquid. As is shown in the drawing, the first liquid composition remains in wellbore 10 at level 16 so as to fluidly communicate with lower productive interval 20 via perforations 14. The second liquid composition is placed above level 16 into level 18 so as to fluidly communicate with productive interval 22 via perforations 14. Subsequently, a third liquid composition, having a density less than the second liquid composition, is placed above level 18 into level 30 so as to communicate fluidly with productive interval 24 via perforations 14. A fourth liquid composition, which is injected into formation 12 by wellbore 10, is placed above level 30 into level 32 so as to fluidly communicate with upper productive interval 26 via perforations 14. All of the liquids which are injected at the different levels comprise a composition which is sufficient to solubilize an alkaline earth metal sulphate deposit from perforations in said wellbore and a productive interval communicating therewith. Once placed in the formation, these liquids at the different levels are allowed to remain in the wellbore so as to contact - 1 - the perforations and intervals communicating therewith which contain the sulphate scale deposit for a time sufficient to solubilize said scale deposits therefrom. The composition comprises an aqueous solution having a pH of from 8 to about 14. Into this solution is placed about 0.1 to about 1.0M of ethylenediaminetetraacetic acid (EDTA) or diethylenetriaminepentaacetic acid (DTPA) , or salts and mixtures thereof, which serves as a chelant. Thereafter, a catalyst is added to the aqueous solution in a concentration of about 0.01 to about 0.5M. The catalyst is selected from a member of the group consisting of oxalate, salicylate, fluoride, persulfate dithionate, hypochlorite, formate, thio, amino, or hydroxy acetate anions. The aqueous liquid composition is used to remove scale from equipment utilized in the production of oil and/or water from an underground formation. Said composition can be utilized to resolve scaling conditions and problems mentioned above. The aqueous liquid composition is directed down the wellbore to remove barium sulphate scale which has fouled the tubular equipment and passage ways. Prior to being directed into the wellbore, the composition may be heated to a temperature between about 25°C to about 100°C. Once within the tubular goods and the passageways requiring treatment, the composition is allowed to remain there for about 10 minutes to about 7 hours. After remaining in contact with the equipment for the desired time, the composition containing the dissolved scale is produced to the surface. This procedure can be repeated as often as required to remove scale from the equipment. US-A-5093020 discloses similar compositions as utilized above for removing barium and strontium sulphate scale deposits. In order to obtain a desired density of the compositions used to solubilize the alkaline earth metal sulphate scales herein, salts of alkali or alkaline-earth metals in aqueous solutions and mixtures thereof may be utilized. The preferred salt solution is sodium chloride because of its ready availability. Zinc chloride solutions may also be used. Although sodium chloride, potassium chloride, calcium chloride, and zinc chloride may be used, bromides of these salts may also be utilized. These salt solutions utilized herein contain a high pH buffering system with a means to obtain a stable pH of about 8 to about 14, preferably about 12. Although an aqueous salt solution is preferred, hydrocarbons can also be utilized to obtain a desired fluid density in combination with the higher density liquid composition used for solubilizing the alkaline earth metal sulphate scale deposits. The preferred pH of the liquids used to solubilize the sulphate scale deposits is about 12. The density of the scale solubilizing composition will be about 12 pounds per gallon. Maintenance of the pH is critical and the density of the liquid in its present form relates to the additive required to maintain the pH. As previously mentioned, the heavier scale solubilizing liquid should be at the bottom of the wellbore with scale solubilizing liquids of a lower density over a higher density solubilizing fluid. The density difference should vary and be about 0.1 wt% difference. By placement of the scale solubilizing liquids of lowering densities proceeding up the wellbore, it is possible to effectively treat an entire zone of a formation containing multiple productive intervals with sections of high permeability i.e. greater than 10 to about 20 md. Because the density is lowered as the liquids proceed up the wellbore, an entire multiple level productive zone can be effectively treated at the low injection rate of about 1 to about 2 BPM. By utilizing this method with the variable density liquids as disclosed, the scale solubilizing liquid can be distributed over an extended perforated interval. A preferred method to be used involves the utilization of two storage tanks at the surface. The scale solubilizing liquid with the highest density is placed in one tank. A second tank which contains a lower density liquid, preferably a saline one, having either a water or hydrocarbon base, is used to obtain a desired density variation. This is accomplished by pumping the salt containing solution along with the sulphate scale solubilizing solution at a desired rate to obtain a density difference of about 0.1 wt%. Following a pre-determined volume of the higher density sulphate scale solubilizing solution, fractions of the lower density salt containing liquid is added to a flow stream directed into the wellbore thereby diluting the concentrated scale solubilizing composition to a lower desired density. A steadily decreasing liquid density allows various portions of the perforated interval to be treated. Salts which are utilized herein are disclosed in US-A-4883124. The density of the first scale solubilizing liquid that is used in a first stage or level is from about 11.5 to about 12.0 pounds per gallon (0.955 to 1.00 kg/dm). The density of the second scale solubilizing solution that will be used in a second stage or level in the wellbore is from about 10.5 to about 11.0 pounds per gallon (0.872 to 0.914 kg/dm). When a third scale solubilizing solution is utilized, this density should be about 9.5 to about 10.5 pounds per gallon (0.789 to 0.872 kg/dm). When an extensive perforated interval is encountered as described in the drawing, a fourth scale solubilizing liquid should have a density of from about 8.5 to about 9.5 pounds per gallon (0.706 to 0.789 kg/dm). In addition to directing or pumping the scale solubilizing liquids into the wellbore, these liquids can be flowed into the wellbore by a concentric tubing arrangement as is disclosed in US-A-4947933. The aqueous medium which is utilized herein can comprise fresh water, brackish water, or seawater, and mixtures thereof. Hydrocarbons which can be utilized herein include fuel oil, kerosene, and mixtures thereof. After the scale solubilizing liquids of varying densities have remained in the wellbore for a time sufficient to solubilize the alkaline-earth metal sulphate scale deposits from perforations and intervals of a formation communicating therewith, the high density scale solubilizing liquid is directed into the wellbore to flush the wellbore out. Afterwards, scale solubilizing liquids with decreasing densities can be flowed into the wellbore to flush out any remaining liquid until a desired lower density of the scale solubilizing liquid remains in the wellbore. Afterwards, a salt solution can be used to flush the scale solubilizing liquid out of the wellbore or it can be circulated out. Thus, the wellbore and perforations will be cleared of the scale solubilizing liquid to a concentration desired. Later, the well can be placed back on production. Subsequently, an enhanced oil recovery (EOR) method can be commenced into the multiple productive intervals so as to remove hydrocarbonaceous from said intervals. Although the present invention has been described with preferred embodiments, it is to be understood that modifications and variations may be made within the scope of the appended claims.

Claims: Claims 1. A method of scale dissolution in a formation containing multiple productive intervals comprising: (a) directing into a perforated wellbore a first liquid composition, of a known density, for dissolving an alkaline-earth metal sulphate scale deposit from perforations and productive intervals communicating with said wellbore; and (b) directing into said wellbore, on top of said first liquid composition, a second liquid composition for dissolving an alkaline-earth metal sulphate scale in a higher perforated interval of said wellbore which communicates fluidly with said productive intervals, wherein said second liquid composition has a density less than that of the first liquid composition. 2. A method according to claim 1, wherein, in step (b) , the density of the second liquid composition is decreased by adding a lower density salt solution of an alkali or alkaline-earth metal. 3. A method according to claim 1, wherein, in step (b) , the density of the second liquid is decreased by adding an aqueous or hydrocarbonaceous solution containing a salt of an alkali or alkaline-earth metal in an amount sufficient to obtain a density of at least 0.1 wt% less than the density of the first liquid composition. 4. A method according to claim 3, wherein said aqueous or hydrocarbonaceous solution contains bromides or chlorides. 5. A method according to claim 2, 3 or 4, wherein the aqueous salt solution is selected from a member of the group consisting of sodium chloride, potassium chloride, zinc chloride, sodium bromide, potassium bromide, or zinc bromide. 6. A method according to any preceding claim, wherein the density of the first liquid composition is from 11.5 to 12.0 pounds per gallon (0.955 to 1.00 kg/dm) . 7. A method according to any preceding claim, wherein the density of the second liquid composition is from 10.5 to 11.0 pounds per gallon (0.872 to 0.914 kg/dm). 8. A method according to any preceding claim, wherein successive liquids with decreasing density of at least 0.1 wt% less than the liquid last added to the wellbore are added above the first and second liquids until all of the desired productive intervals and perforations communicating therewith have been treated with the scale solubilizing solution. 9. A method according to any one of claims 1 to 7, wherein, after step (b) , a third liquid composition for dissolving scale is directed into the wellbore which has a density from 9.5 to 10.0 pounds per gallon (0.789 to 0.831 kg/dm) . 10. A method according to claim 9, wherein, after step (b) , a fourth liquid composition for dissolving scale is directed into the wellbore which has a density from 8.5 to 9.0 pounds per gallon (0.706 to 0.748 kg/dm) . 11. A method according to any preceding claim, wherein the first and second liquid compositions comprise an aqueous solution having a pH from 8 to 14, each solution containing: (i) an aminocarboxylic acid or a polyamine chelant, salts and mixtures thereof in an amount from 0.1 to 1.0M; and (ii) a catalyst in an amount from 0.01 to 0.5M which catalyst is a member selected from the group consisting of fluoride, oxalate, persulfate, dithionate, hypochlorite, formate, thio, a ino, and hydroxy acetate anions, thereby making a solution which can dissolve substantially more scale within a substantially reduced time than is possible with said chelant alone.

Inventors: FIELER ELEANOR ROTH, JENNINGS ALFRED ROY JR, PAUL JAMES MARK, FIELER, ELEANOR, ROTH, JENNINGS, ALFRED, ROY, JR., PAUL, JAMES, MARK
Applicants: MOBIL OIL CORP, MOBIL OIL CORPORATION
Application year: 1994
Main classification: E21B37
IPC codes: C09K8, E21B37